This is a book written by several editors/sensitivity readers of salt and sage.
It does what it says it does, which is enlighten authors (and readers!) about harmful tropes/stereotypes often associated with Black characters, as well as some pointers about how to write about some topics that are quite specific to the Black experience.
It's important for readers, I think, to recognize and be educated about harmful stereotypes, racism, xenophobia and bigotry, so that one can recognize it as a bias the author holds.
Sensitivity readers exist because there are topics which we are more sensitive about and topics we are less so. For instance, women would probably do better spotting a misogynistic plot and characterization, than men. But with awareness and education, both men and women can easily spot misogynism in books and movies. I read this book because I want to easily spot anti-black racism in books. While some stereotypes were already known to me ("Magical Negro" "Uncle Tom" "Mammy" "Angry Black Woman"), others were less obvious to me and I only had a superficial understanding of them.
Now, this book isn't exhaustive in any way. In fact, authors would have to do a lot more research than this to be able to write authentic black (main) characters without making a complete mess out of it. For readers, though, this is a really good guideline so you can spot racist nonsense and point it out in your review so that POC reviewers don't have to shout at the top of their lungs about somethings when it's glaringly obvious.